1. Field of the Invention
This invention relates to sharpness processing apparatus used, for example, in a video plate-making process for enhancing or suppressing tone variations in outline portions of an original image.
2. Description of the Related Art
The principle of sharpness processing will be described with reference to FIG. 1.
Assume that a particular region of an original image has an image signal S (i.e. an original image S) as shown in FIG. 1 (A). Key pixels in the particular region are successively selected, and an average signal is obtained from color component signals, for example, of 5.times.5 pixels surrounding each key pixel. This corresponds to an unsharp signal U as shown in FIG. 1 (B).
Next, the unsharp signal U is subtracted from the original signal S to obtain an unsharp mask signal (S-U) as shown in FIG. 1 (C). The unsharp mask signal (S-U) is added to the original signal S to obtain a signal enhancing outline portions as shown in FIG. 1 (D). At this time, the unsharp mask signal (S-U) is multiplied by a parameter k to adjust a degree of processing. The original signal S consists of three color component signals (RGB signals). Consequently, the sharpness processing is expressed as follows:
S.sub.R +k (S-U)
S.sub.G +k (S-U)
S.sub.B +k (S-U)
A signal with enhanced sharpness is obtained if parameter k is larger than zero. However, if -1&lt;k&lt;0, the signal will be something between the original signal S and unsharp signal U as seen from the above expressions. This is a signal having reduced sharpness opposite to enhancement; a signal having undergone what is known as smoothing processing.
When such sharpness enhancement processing (or suppression processing: these are collectively called sharpness processing) is effected only for a particularly region in an original image, a pointing device such as a mouse is operated to designate the region on a monitor screen displaying the original image, and the above processing is carried out only for the pixels within this region.
However, a very difficult and irksome operation is required for designating such a particular region.
In the first place, where sharpness processing is desired only for a particular graphic figure, the more complicated the image is, the longer time is spent in designating the region. Where sharpness processing is desired only for graphic figures of a particularly color in the original image, the graphic figures having its color components must be designated individually. This involves a great amount of work.
When a particular region is designated in an original image having gradually changing image signals throughout, the image tends to be converted into an unnatural image with boundaries of the region abruptly becoming sharp or unsharp.